public sealed class BeforeSketchCompletedEventArgs : System.EventArgs 

// Occurs before a sketch is completed. BeforeSketchCompleted event is fired by 
//  - COTS construction tools (except annotation, dimension geometry types), 
//  - Edit Vertices, Reshape, Align Features
//  - 3rd party tools with FireSketchEvents = true
// Register for the BeforeSketchCompleted event
BeforeSketchCompletedEvent.Subscribe(beforeSketchCompletedEventArgs
  =>
{
  //assign sketch Z values from default surface and set the sketch geometry
  var modifiedSketch = beforeSketchCompletedEventArgs.MapView.Map.GetZsFromSurfaceAsync(beforeSketchCompletedEventArgs.Sketch).Result;
  beforeSketchCompletedEventArgs.SetSketchGeometry(modifiedSketch.Geometry);
  return Task.CompletedTask;
});